For industrial ethylene hydroformylation processes, the catalyst of choice typically comprises rhodium metal promoted with triphenylphosphine (TPP). Rhodium-TPP is a proven, dependable technology capable of delivering excellent production rates, but maintaining these high rates throughout the lifetime of the catalyst requires high reaction temperatures. In hydroformylation processes, high temperature promotes aldol condensation, and this in turn lowers olefin efficiency and produces high boiling by-products. The ever increasing reactor volume taken up by these “heavies” can ultimately limit the effective life of the catalyst. Maintaining production rates in a rhodium-TPP process also requires relatively large amounts of rhodium, which may add significantly to the overall cost of the process. Thus a catalyst capable of maintaining high production rates at lower temperatures and/or with less rhodium would improve process economics.
The activity of a rhodium-catalyzed hydroformylation process is determined to a large extent by the ligand employed, but it is also dependent on the olefin substrate. For example, it is well known that terminal olefins are much more reactive than internal olefins. While most rhodium-ligand combinations will demonstrate reasonable reaction rates for ethylene hydroformylation, the absolute reaction rate of a given catalyst is not readily predicted based on results obtained with other olefins. For example, many rhodium-ligand combinations that give very high reaction rates for propylene hydroformylation are only moderately active for ethylene hydroformylation. The reason for these observed relative differences is not understood, however the fact remains that the activity of a rhodium catalyst promoted by a particular ligand for the hydroformylation of other olefins is not predictive for the hydroformylation of ethylene.